The circuit structure for a conventional interferometer type polarization beam combiner and splitter is shown in FIG. 1 (see patent literature 1). This interferometer type polarization beam combiner and splitter is a Mach-Zehnder interferometer that includes two optical couplers 111 and 112 and two optical waveguides 121 and 122. The operation of the Mach-Zehnder interferometer is changed as follows, depending on an optical path length difference that has been set for the two optical waveguides.